The importance of achieving accurate mutual alignment of individual components in any optical system is well known. The miniature dimensions of components used in modern optical communication systems render such alignment difficult both to achieve and to maintain. For example, one problem in the construction of laser transmitters is that of efficiently coupling the optical output signal from a laser diode into an optical fiber. To obtain efficient coupling, the laser is desirably precisely aligned with the acceptance area of the optical fiber. When such alignment is achieved, the laser is then fixed in place, ideally by a method that ensures alignment is sustained throughout the device lifetime.
Typically, fiber-coupled diode lasers are packaged in gold plated metal packages and the fiber is held in alignment with the laser using either epoxy, laser weld, or solder attachment techniques. Epoxy attachment is low cost but is not reliable over a long period of time due to outgassing and alignment shifts arising from aging and temperature cycling. Laser weld techniques are reliable but require costly ferrulization of the fiber and specially designed mounts or clips to allow weld attachment of the ferrulized fiber. Solder attachment techniques, on the other hand, are reliable and low cost, and have become prevalent in the art.
The mounting point at which the fiber is soldered desirably has specific material properties in order to work effectively. An acceptable material for the mounting point desirably has a low thermal conductivity (e.g. less than 50 W/m-K) and a thermal expansion coefficient that maintains fiber alignment while the package is heated. The exact thermal expansion property desired may depend on the material to which the laser is mounted, the respective thickness of the fiber mount and laser submount, and/or the temperature profiles expected during operation. The fiber mount material also may be soldered or be able to be plated with a solderable material. During the soldering process, the fiber mount may experience significant stress resulting from differential expansion due to temperature gradients and materials differences. Therefore, the fiber mount desirably has a high tensile strength (e.g. greater than 25 kpsi) to avoid fracturing.
Currently, fiber-coupled laser packages include separate mount components for the fiber and the laser and primarily have a snout feedthrough for the fiber to enter the package body. This feedthrough requires the fiber to be threaded through at some point in the assembly process, which may cause manufacturing complications, as feeding a fiber through a side tube on a package may be difficult to implement with standard automated manufacturing equipment. Since the associated feedthrough step includes horizontal threading of the optical fiber, it is difficult to automate and causes yield issues associated with fiber damage.
During manufacture of fiber-coupled laser packages, it is also desirable to monitor the alignment of the optical fiber with the laser using cameras and machine vision. Desirable views may include an overhead view of the process as well as side views of the process. In many of the currently used packages, however, it is difficult to obtain a side view of the fiber-laser alignment area, which presents complications to automation monitoring.